The 15 default User Defined Field Names appear in the following order:
User1
User10
User11
User12
User13
User14
User15
User2
User3
User4
User5
User6
User7
User8
User9
It is suggested you names them like this:
To do this, go in to ACT!
Then go to Edit, Define Fields
Then they would look like this on the screen.